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News analysis Australians are "complacent" to the risks posed by earthquakes and that one could strike a major city, say earthquake experts.

The warning comes after two moderate-sized earthquakes recently struck the Gippsland town of Korumburra in southeast Victoria. Both were felt 120 kilometres away in the city of Melbourne.

The earthquakes registered magnitude 4.6 on the Richter scale, with another small earthquake felt in the area in January.

Both struck 15 kilometres below ground and were associated with uplift of the Strzelecki Ranges.

Senior seismologist Clive Colins of Geoscience Australia says extra seismographs have been placed in the region to monitor aftershocks.

He says the activity isn't unusual for the area and believes it will die down.

But he warns similar or larger earthquakes could strike more populated areas in the future.

"Our main concern is that one gets complacent about earthquakes in Australia," says Collins. "You have to be prepared that you can get a moderate earthquake of this size in an urban area, such as [the earthquake] in Newcastle in 1989."

That earthquake, which was registered as a magnitude 5.6 earthquake, caused AU$1 billion worth of damage and killed 13 people.

"That was a real wake up call," he says.

Cities vulnerable

Seismologist Dr Kevin McCue of Central Queensland University in Rockhampton, says Australia is hit by a magnitude 6 earthquake every five to six years.

"I think that would shock most people," he says. "Currently, one is overdue, so we're just waiting to see what will happen in Victoria."

But, McCue says predicting them is problematic.

"I think it is just luck we haven't had [earthquakes] under Melbourne and Sydney," he says. "Most cities are near earthquake [prone areas], it's just people won't believe it until they've had one near their city. This is a real problem."

Colins agrees, pointing out that most infrastructure in Australia isn't designed to cope with a major earthquake.

"Brick and masonry buildings in Australia are not designed for a major earthquake and in many cases [the buildings] are very old," says Colins.

"While there may not be high rises and dams falling down, the older, smaller buildings that are everywhere in Australia are very vulnerable. If a moderate earthquake happened in Sydney, Perth or Adelaide it would be a major disaster."

Squeezed from the sides

Earthquakes frequently occur close to plate boundaries, where the plates that make up the earth's crust push and slide against each other.

Despite sitting in the middle of a tectonic plate, scientists say Australia is subjected to the stresses and strains from movements at the edges of plate boundaries.

McCue says, compared to other continents that also sit inside plate boundaries, Australia is relatively earthquake-prone.

"Compared to Canada, US, South Africa, central Africa and India, Australia is more active," he says.

US seismologist Professor Paul Somerville, deputy director of Risk Frontiers, based at Macquarie University in Sydney, says Australia is under "quite high tectonic stress".

"As is the case in other stable regions, the earthquake activity appears to be generally higher around the margins (edges) of the continent than in its interior," he says.

"Since Australia's population is more concentrated on its coasts than other stable regions, this by itself presents a higher hazard level."

Short history

Australia's recorded earthquake history is relatively short, which may be lulling residents into a false sense of security.

Earthquake risk is calculated by counting the number of quakes that have occurred in a region to determine the probability another will strike in the future.

Australian and New Zealand building code for earthquakes is largely based on these forecasts.

For example, Gippsland is expected to experience six or seven magnitude 4 earthquakes, and three to five magnitude 4.5 earthquakes, in the next 20 years.

But experts warn these estimates may be misleading as Australia's earthquake record only goes back 130 years.

They say, some faults may only rupture once every thousand or tens of thousands of years.

"The most important thing is to do more work on identifying active faults in Australia," says Sommerville.

"There are probably lots of active faults that could generate earthquakes in places that haven't had earthquakes yet."